Rotary filtration started with a design of Werner & Pfleider in Germany. The basic concept continued in subsequent designs of Rehau and Gneuss, also German firms. Most recently Patt Filtration in Canada has further improved and produced equipment for rotary filtration. This invention builds upon the wave filter design of U.S. Pat. No. 6,582,598 filed Dec. 14, 2000 which doubled the available filter area within a given size filter by again more than doubling the wave filter area.
A normal or typical rotary filter is used to provide continuous filtration of materials with a constant pressure drop through the filter. It consists of a rotatable disc clamped between two blocks or side plates which have flow through channels in each. Flow is from an input block through the flow through shaped and expanded area channels which communicates between the input side, through a portion of an annular filter support and filter media containing area of the disc, and to and through a similarly shaped expanded area exit side flow channel to further processing equipment. The rotation of the disc brings new, non-contaminated filter media and the associated filter supports from the flow channels into the flow through area of the filter and filter support. At the same time, as the disc rotates, a small amount of material is removed with the associated contaminated filter media and filter support. Sealing is metal to metal making a properly constructed filter leak free. The screens or filter media used is held in place by edge friction or by holding rings but the design does not permit backward flow and in some cases retaining bars are needed to prevent backward movement of the filter media. Any such backward movement would result in jamming of the filter media in the metal to metal seal surfaces surrounding the filter which are the lands separating filter media containing chambers. Such jamming can damage the leak free seals.
The rotary filter, while an established technology, is poorly understood. A series of factors such as venting of filter channels, the hardening and the coatings needed to provide corrosion and antigalling protection, the forces used for clamping the parts together to form a seal, and the method to drive the rotary motion are all being changed as the knowledge of the system function improves. Since the filter is very expensive and involves extreme machining accuracy, methods to expand the filtration capacity of a given size filter is especially important. There has been little improvement in the filtration capacity of a disc of a given size until the above mentioned Wave Filter was developed and since then the improvements in filter area have again stalled. Despite the lack of progress, there is a continuing need to improve flow capacity of the filters.
A rotary filter has unique advantages for many applications. It is usually used for high viscosity materials and these are often at high temperatures. A typical use is to filter molten polymers at temperatures of 350 to 750 degrees Fahrenheit and pressures of 2,000 to 12,000 pounds per square inch. The properly designed and produced rotary filter has essentially no leakage at the molten polymer conditions. It also provides essentially no pressure variation within the process stream as the filter media is changed to bring new filter media into the flow stream of material (such as molten polymers) needing filtration since the lands between filter chambers are always present in a constant area within the fluid flow path thus evening the effect to changing from dirty filter chambers to clean filter chambers. It also permits incremental changes in the filter chamber advance rate so that the contaminant load on the filters may be kept relatively constant despite varying contaminant loading in the fluid being filtered. The rotary filter further resists high temperatures and pressures of flow streams being filtered. The downside of all of these advantages is that costly materials and machining methods are essential to the success of the rotary filter, making it an expensive filter.
While the rotary filter has many advantages, one big disadvantage is that the area for filtration is highly constrained. The filter media is placed in flat arrays within filter chambers machined in the rotating filter disc. The filter area (chamber area) can be, at its maximum, only a small sector of the annular ring formed within a filter disc. In numerical terms, in a few small filters the ratio of filter area in the flow stream being filtered to the total area of filter chambers on the disc may be as low as 1/25. In most lower pressure rotary filter applications the maximum of filter area in the flow stream to total filter area of filter chambers on the disc is ⅕. In the usual high viscosity filtration use, the filter construction is further constrained by the need for support of the filter media. Due to very high flow stream pressures substantial support plates, typically a thick plate with drilled flow through holes (in contrast to many cartridge filters which handle low pressures and have very thin supports or no supports) are needed. The balance of internal forces and the need to access the filter support to replace contaminated filter media usually restricts the surface area within the flow stream to no more than about 20% of the annular ring area. There is a need for higher flow area contact with filter media in rotary filters.
In a filter handling a viscous material, the filter media, which may have capacity of filtering particles ranging from tens of thousandths of an inch in diameter to microns in diameter and which may consist of several layers of filter media such as screen or sintered metal strip, has by far the greatest resistance to flow of the materials being filtered. In the most common applications of rotary filters, the filtrate is a molten polymer and the stainless mesh screen filter media has mesh of 400 to 20 mesh depending on the application. These screens are stacked together to remove contaminants and so that the finer screens are reinforced by the thicker screens to prevent screen breakage. Use of sintered strip has been very limited due to the rapid clogging of this very expensive filter media and the high pressure drop through most sintered filter media.
The filter support and the chamber that hold the filter media are also potential problems. There is a desire to minimize the volume of the filter chamber to minimize the amount of fill in each chamber which affects fill time and so flow rates through each part of the chamber are sufficient to prevent time/temperature degradation of the filtrate (the fluid being filtered). This means that the depth of the filter chamber and especially its area are key design considerations. It is beneficial to minimize both depth and area of the filter chamber for flow reasons, however, the maximum area is beneficial to lowered total pressure drop and to increased filter capacity. In addition, the filter support, which is a plethora of small drilled holes, typically in the 2 mm to 5.5 mm diameter range, causes some resistance to flow. These small holes create a pressure drop that increases as hole diameter decreases and as hole depth and total flow increases. The depth of the holes in the filter support also is a function of the needed stiffness (which is a function of material and thickness) of the rotary filter disc. Looking at all of these factors, typical disc thicknesses are in the range of 20 to 50 mm with the required stiffness being a predominate controlling factor that keeps the disc thickness high despite the adverse pressure effects due to thickness. Greater thicknesses of the disc and the resulting improved stiffness and lowered distortion would mean either larger free area in the filter chamber and the resulting holdup of filtrate or higher pressures due to longer drilled hole lengths in the support plate. There is a need for a method to increase disc thickness without increasing free area (hangup) or pressure.
The steel used in production of the rotary filter disc is also an expensive high alloy specialty steel. The cost of the steel suggests that the rotary disc is kept to a minimum thickness, typically around an inch thick. First and foremost, the disc must be totally distortion free. Flatness of both sides and the parallelism of the two sides must be within microns. Increasing disc thickness increases disc cost but makes the disc more resistant to distortion.
The active filter areas within the rotary filter have been flat areas within the rotary disc. Specifically filter media chambers are machined into the filter disc and are separated by webs or lands that effect seals with the body plates surrounding the filter containing rotary disc. These filter chambers are shallow flat bottomed areas with the filter support and through holes in this flat bottom area. Filter media is placed into the chambers, resting on the flat surface of the bottom of the chambers and held into place by friction with the shallow walls of the chamber. The patented advance of the wave filter allowed greater filter area by corrugating the filter media and machining the same wave or corrugation shape in the filter support so that the filter media was totally supported and thus did not distort despite the filter media wave like shape
Flat filter supports and flat filter media has been used in all rotary filters except the wave filter because flat filter media is cut so that there is an effective slight interference with the filter chamber walls. This slight interference holds the filter media in place by interaction with the side walls of the filter chamber. Holding the filter media in place in this way insures that the filter media never rises higher than the depth of the filter chamber. The filter media is thus frictionally prevented form moving or falling into a position where the filter media can become wedged into the metal to metal seal between the rotary filter disc and the blocks that constrain the rotary disc. This is important because the tight metal to metal seals of a rotary filter can be destroyed if the filter media is caught between the filter disc and the side plate blocks that constrain the internal flow pressure of the fluid being filtered as the rotary disc moves. An improved method that provides a positive grip to restrain movement of filter media would be an improvement in rotary filtration.
There remains a need for finer filter media, greater filter media area for a given size filter and for economic use of filter media.